Sunday, January 11, 2015

Creating Eucalyptus Machine Images from a Running VM

I often use Eucalyptus private cloud platform for my research. And very often I need to start Linux VMs in Eucalyptus, and install a whole stack of software on them. This involves a lot of repetitive work, so in order to save time I prefer creating machine images (EMIs) from fully configured VMs. This post outlines the steps one should follow to create an EMI from a VM running in Eucalyptus (tested on Ubuntu Lucid and Precise VMs).

Step 1: SSH into the VM running in Eucalyptus, if you already haven't.

Step 2: Run euca-bundle-vol command to create an image file (snapshot) from the VM's root file system.
euca-bundle-vol -p root -d /mnt -s 10240
Here "-p" is the name you wish to give to the image file. "-s" is the size of the image in megabytes. In the above example, this is set to 10GB, which also happens to be the largest acceptable value for "-s" argument. "-d" is the directory in which the image file should be placed. Make sure this directory has enough free space to accommodate the image size specified in "-s". 
This command may take several minutes to execute. For a 10GB image, it may take around 3 to 8 minutes. When completed, check the contents of the directory specified in argument "-d". You will see an XML manifest file and a number of image part files in there.

Step 3: Upload the image file to the Eucalyptus cloud using the euca-upload-bundle command.
euca-upload-bundle -b my-test-image -m /mnt/root.manifest.xml
Here "-b" is the name of the bucket (in Walrus key-value store) to which the image file should be uploaded. You don't have to create the bucket beforehand. This command will create the bucket if it doesn't already exist. "-m" should point to the XML manifest file generated in the previous step.
This command requires certain environment variables to be exported (primarily access keys and certificate paths). The easiest way to do that is to copy your eucarc file and the associated keys into the VM and source the eucarc file into the environment.
This command also may take several minutes to complete. At the end, it will output a string of the form "bucket-name/manifest-file-name".

Step 4: Register the newly uploaded image file with Eucalyptus.
euca-register my-test-image/root.manifest.xml
The only parameter required here is the "bucket-name/manifest-file-name" string returned from the previous step. I've noticed that in some cases, running this command from the VM in Eucalyptus doesn't work (you will get an error saying 404 not found). In that case you can simply run the command from somewhere else -- somewhere outside the Eucalyptus cloud. If all goes well, the command will return with an EMI ID. At this point you can launch instances of your image using the euca-run-instances command.

Friday, January 2, 2015

Developing Web Services with Go

Golang facilitates implementing powerful web applications and services using a very small amount of code. It can be used to implement both HTML rendering webapps as well as XML/JSON rendering web APIs. In this post, I'm going to demonstrate how easy it is to implement a simple JSON-based web service using Go. We are going to implement a simple addition service, that takes two integers as the input, and returns their sum as the output.
package main

import (

type addReq struct {
        Arg1,Arg2 int

type addResp struct {
        Sum int

func addHandler(w http.ResponseWriter, r *http.Request) {
        decoder := json.NewDecoder(r.Body)
        var req addReq
        if err := decoder.Decode(&req); err != nil {
                http.Error(w, err.Error(), http.StatusInternalServerError)
        jsonString, err := json.Marshal(addResp{Sum: req.Arg1 + req.Arg2})
        if err != nil {
                http.Error(w, err.Error(), http.StatusInternalServerError)
        w.Header().Set("Content-Type", "application/json")

func main() {
        http.HandleFunc("/add", addHandler)
        http.ListenAndServe(":8080", nil)
Lets review the code from top to bottom. First we need to import the JSON and HTTP packages into our code. The JSON package provides the functions for parsing and marshaling JSON messages. The HTTP package enables processing HTTP requests. Then we define two data types (addReq and addResp) to represent the incoming JSON request and the outgoing JSON response. Note how addReq contains two integers (Arg1, Arg2) for the two input values, and addResp contains only one integer (Sum) for holding the total.
Next we define what is called a HTTP handler function which implements the logic of our web service. This function simply parses the incoming request, and populates an instance of the addReq struct. Then it creates an instance of the addResp struct, and serializes it into JSON. The resulting JSON string is then written out using the http.ResponseWriter object.
Finally, we have a main function that ties everything together, and starts executing the web service. This main function, simply registers our HTTP handler with the "/add" URL context, and starts an HTTP server on port 8080. This means any requests sent to the "/add" URL will be dispatched to the addHandler function for processing.
That's all there's to it. You may compile and run the program to try it out. Use Curl as follows to send a test request.
curl -v -X POST -d '{"Arg1":5, "Arg2":4}' http://localhost:8080/add
You will get a JSON response back with the total.